feat(backend): Add Celery worker infrastructure with task stubs

- Add Celery app configuration with Redis broker/backend
- Add task modules: agent, workflow, cost, git, sync
- Add task stubs for:
  - Agent execution (spawn, heartbeat, terminate)
  - Workflow orchestration (start sprint, checkpoint, code review)
  - Cost tracking (record usage, calculate, generate report)
  - Git operations (clone, commit, push, sync)
  - External sync (import issues, export updates)
- Add task tests directory structure
- Configure for production-ready Celery setup

Implements #18

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/tests/tasks/__init__.py

@@ -0,0 +1,11 @@
+# tests/tasks/__init__.py
+"""
+Tests for Celery background tasks.
+
+This module tests the Celery configuration and all task modules:
+- agent: Agent execution tasks
+- git: Git operation tasks
+- sync: Issue synchronization tasks
+- workflow: Workflow state management tasks
+- cost: Cost tracking and aggregation tasks
+"""